Micro Electro Mechanical Systems (MEMS) generally relate to the integration of mechanical elements, sensors, actuators, and/or electronics onto a support structure, such as a silicon substrate or wafer. Some common examples of MEMS devices include miniature engines, laser beam splitters, air bag accelerometers, mirror arrays, and ink jet heads for printers. Many of these systems have device structures or components that are moveable, or that must otherwise be detached from the MEMS device support structure to function properly.
A significant difficulty in manufacturing MEMS devices is “stiction,” which is the adhesion of a micro movable component to another structure, such as a support substrate. Stiction occurs in a MEMS device when a component on the device has been detached from its support structure, and surface tension of a surrounding fluid used in the manufacturing process distorts the component so that the components bend and stick to one or more other surfaces. If stiction occurs, the MEMS device is generally defective. The device must then be repaired, which can be expensive and time-consuming, or it is discarded.
Aqueous liquids, which are typically used in the manufacturing of MEMS devices, have a strong tendency to cause stiction. Consequently, in the manufacture of MEMS devices, efforts are generally made to perform a “release etch” of the moveable components on the device, and to perform all subsequent processing as well, in a way which will avoid stiction. Techniques commonly used have included the use of plasma etches, vapor etches, and supercritical fluid exposure.
Hydrofluoric acid (HF) vapor etching has been used to perform the release etch, without causing stiction. In the case of fluorine based etchants, such as HF, the release layer to be etched is usually silicon dioxide. The key to successful vapor phase etching, without creating stiction, is to control the formation of a condensate film that forms on the MEMS device. Vapor phase etching occurs as the vapor condenses on the device surface. If too much condensate forms, a liquid boundary layer on the device will become thick enough to cause stiction. If insufficient condensate forms, conversely, the etch rate will typically be too low for practical application in device manufacture, and process uniformity may be poor since the boundary layer is not fully developed, resulting in large variations across the wafer and from one wafer to another.
While HF vapor has been successful at etching silicon dioxide films, there are often other materials present during the etch. Some of these may etch to a greater or lesser degree, and may contribute to the formation of other reaction products, which are often undesirable. In general, a desired reaction for etching silicon dioxide with HF is:4HF+SiO2→SiF4+2H2O
Control of the etching rate is important to ensure that excess water does not form so rapidly that it interferes with the etching process. Since SiF4 is a gas, the etching process may proceed without being impeded by the formation of solids or liquids, which would otherwise interfere with mass transport mechanisms. However, other elements that might be present include dopants, such as boron, phosphorous, and/or arsenic, as well as materials such as silicon nitride. Competing side-reactions might also form compounds such as H2SiF6, or other similar compounds.
In many cases, the formation of these additional compounds or contaminants will interfere with the desired etching process to an unacceptable degree. Moreover, these contaminants may render the MEMS devices damaged or completely inoperable. It is typically time-consuming and expensive to effectively clean and/or repair a MEMS device that has been damaged by contaminants. Accordingly, there is an important need for better methods for manufacturing MEMS devices.